Question

(3.25) Calculate the molar mass of a compound if 0.372 mole of it has a mass...

(3.25) Calculate the molar mass of a compound if 0.372 mole of it has a mass of 152 g. (Hint: if 0.372 mole is to 152 g, how many grams will 1 mole have if you set this equation by ratio & proportion)

Caffeine is found in your coffee and your tea. Its molecular formula is C8H10N4O2. (a) Calculate the molar mass of caffeine. (b) Calculate the percent composition by mass of C, H, N, and O in caffeine. (c) Red bull contains 80 mg of caffeine. How many molecules of caffeine are there in red bull ? [C 12.011 amu; H 1.008 amu; N 14.008 amu; O 15.999 amu]

Homework Answers

Answer #1

molar mass = (given mass)/(number of mole)
= 152/0.372
= 409 g/mol

a)
molar mass of C8H10N4O2 = (8*12.011)+(10*1.008)+(4*14.008)+(2*15.999)
= 96.088+10.08+56.032+31.998
= 194.198 amu
b)
%mass = {(mass of element)/(total mass)}*100
%mass of C = (96.088/194.198)*100
= 49.5%
%mass of H = (10.08/194.198)*100
= 5.2%
%mass of N = (56.032/194.198)*100
= 28.8%
%mass of O = (31.998/194.198)*100
= 16.5%

c)
given mass = 80 mg
= (80*10^-3) g
number of mole of caffeine = (given mass)/(molar mass)
= (80*10^-3)/(194.198)
= (0.412*10^-3) mole

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